Method and apparatus for measuring transverse thickness profile of a metal strip

Abstract

The invention has an X-ray emitting tube and a linear detector on opposite sides of a plane containing a strip to be measured and an electronic acquisition, measuring and control unit. A mask transversely moveable relative to the strip occludes radiation from the X-ray emitting tube which is not intercepted by the strip or which passes through the edge region of the strip. This device offers an improved degree of precision measurement near the edge of the strip.

What is claimed is: 
     
       1. An apparatus for measuring a thickness profile of a metal strip by radiation absorption comprising: a radiation emitting tube disposed on a side of a plane substantially defined by said strip;   a linear radiation detector disposed on a side of said plane opposite said tube for generating a signal profile representative of an amount of radiation incident thereon;   a mask interposed between said tube and said detector to partly occult the radiation emitted by said tube and mask a corresponding part of said detector;   determining means for receiving said signal profile and determining a first position at which said profile changes from a first level representing radiation occulted only by said strip to a second level representing radiation occulted by said strip and by said mask, and for determining a second position at which said signal profile changes from said second level to a third level representing radiation occulted only by said mask;   difference means for generating a signal indicative of a difference between said first and second positions; and   control means responsive to said difference signal for controlling a position of said mask to maintain said difference at a predetermined amount.   
     
     
       2. An apparatus according to claim 1, wherein said mask is located between said strip and said detector. 
     
     
       3. An apparatus according to claim 2, wherein: an edge of said mask is disposed to cover a region of said strip adjacent to an edge of said strip, said strip edge region having a predetermined width; and   said mask is disposed to occult radiation from said tube not occulted by said strip and radiation from said tube occulted by said strip edge region.   
     
     
       4. An apparatus according to claim 3, further comprising: a support moveable in a direction perpendicular to said edge of said strip, said mask being disposed on said support; wherein   said control means controls said mask position by moving said support.   
     
     
       5. An apparatus according to claim 1, wherein said mask is located between said tube and said strip. 
     
     
       6. An apparatus according to claim 3, wherein: said mask is disposed to occult radiation from said tube not occulted by said strip and radiation from said tube occulted by a region of said strip adjacent to an edge of said strip, aid strip edge region having a predetermined width.   
     
     
       7. An apparatus according to claim 6, further comprising: a support moveable in a direction perpendicular to said edge of said strip, said mask being disposed on said support; wherein   said control means controls said mask position by moving said support.   
     
     
       8. An apparatus according to claim 1, wherein said radiation emitted by said tube is X-ray radiation. 
     
     
       9. An apparatus according to claim 8, said linear detector comprising: an array of photodetectors; and   a scintillating substance disposed between said tube and said array.   
     
     
       10. An apparatus according to claim 1, further comprising reference means responsive to a portion of said thickness profile at said third level for generating a reference thickness signal. 
     
     
       11. An apparatus according to claim 10, wherein said mask comprises a plurality of steps, each of said steps having a different thickness. 
     
     
       12. An apparatus according to claim 1, further comprising: a fixed frame;   first guiding slideways disposed in an upper part of said frame;   second guiding slideways disposed in a lower part of said frame;   a first carriage carrying said tube, said first carriage being slidably mounted on said first guiding slideways;   a second carriage carrying said linear detector, said second carriage being slidably mounted on said second slideways;   a unit;   third slideways slidably mounted in said unit, said third slideways carrying said mask, and extending in a direction perpendicular to a longitudinal direction of said strip; and   motors for controlling displacements of said carriages and said mask.   
     
     
       13. An apparatus according to claim 1, wherein said mask has a composition similar to that of said strip. 
     
     
       14. A method for measuring a thickness profile of a metal strip by radiation absorption comprising the steps of: providing a radiation emitting tube and a linear radiation detector on opposite sides of a plane substantially defined by a strip to be measured;   providing a mask between said tube and said detector so that the mask occults radiation from said tube not occulted by said strip and radiation from said tube occulted by a region of said strip proximate to an edge of said strip, said strip edge region having a predetermined width;   irradiating said detector with radiation from said tube to generate a thickness profile from said detector representative of radiation incident thereon;   determining a first position at which said profile changes from a first level representing radiation occulted only by said strip to a second level representing radiation occulted by said strip and by said mask;   determining a second position at which said signal profile changes from said second level to a third level representing radiation occulted only by said mask;   determining a difference between said first and second positions; and   controlling a position of said mask to maintain said difference at a predetermined amount.   
     
     
       15. A method according to claim 14, further comprising a step of generating a reference thickness signal responsive to a portion of said thickness profile at said third level.